In situ Raman experiments excited with 244 nm radiation are carried out on Pt/γ-Al 2 O 3 and Pt/Ba/γ-Al 2 O 3 catalysts. The addition of Ba allows the normal catalyst to store substantial amounts of NO x as Ba nitrates under lean burn conditions. Dilute amounts of NO and/or SO 2 plus O 2 in N 2 are flowed over the catalysts, with the temperatures and concentrations chosen to simulate the NO x trapping and sulfur poisoning characteristics encountered in normal operation. Adsorbed species observed on Pt/γ-Al 2 O 3 are nitrite/nitro species and sulfate on γ-Al 2 O 3 and NO on Pt. On Pt/Ba/γ-Al 2 O 3, in situ UV Raman spectra showed typical NO x trap behavior: NO x and SO x are stored as Ba(NO 3 ) 2 and BaSO 4 during lean conditions and released at higher temperatures in H 2 flow. Prolonged SO 2 exposure eventually deactivates the catalyst for NO x storage. The sulfate purged by heating to 500 °C in H 2 is not completely removed from the catalyst, since it can reappear at lower temperature without additional exposure to SO 2 . On both Pt/γ-Al 2 O 3 and Pt/Ba/γ-Al 2 O 3, NO x species form more quickly than sulfates under our reaction conditions, but the sulfates are more stable and eventually block NO x adsorption, since they occupy the same sites.
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Uy et al. (2001) studied this question.
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